New hull design won't rock the boat

By Edd Gent

Published Thursday, September 5, 2013

A new hotel ship design won’t rock thanks to opposing waves created in specially-designed tanks fitted in the hull.

The ship, designed by the Norwegian maritime design company SALT, will house up to 800 personnel working on offshore installations as an alternative to the semi-submersible platforms commonly used at present.

In addition to the special hull design, the ship is equipped with six so-called “azimuth thrusters” – propeller drives with directional control used to keep the ship in the same position when in hotel mode by opposing the external forces from waves, currents and wind, but can also be used to provide forward propulsion.

“The aim is to create a more mobile hotel unit which can be leased by oil companies that operate in several parts of the world. Our job has been to test and document the ship characteristics and investigate whether the characteristic makes for a comfortable stay at sea,” said Sverre Anders Alterskjær at MARINTEK, a subsidiary of independent research organisation SINTEF.

The integrated system designed to reduce rolling of the vessel has been developed by Hoppe Marine and tested by Alterskjær and his research colleagues at MARINTEK using the test basins in their laboratories.

The roll damping is achieved using tanks integrated into the bottom and sides of the hull – called “U-tanks” because of their shape.

“The tanks are filled with water which is set in motion in opposing phase to the wave forces acting on the hull,” Alterskjær said.

The tanks are fitted with air valves at the top which partially control the water motion in the tanks and valve opening can be adjusted depending on the ships roll period, resulting in reduced rolling and improved comfort for those on board.

The researchers investigated the optimal opening of the valves at the top of the U-tanks and the correct water level relative to the ship design, as well as measurement of the motion of the ship during voyages and in hotel mode.

“The wide-ranging tests have also revealed that the hull design functions optimally and that the ship has the characteristics necessary to handle this type of operation,” said Johannes Eldøy, a marine designer at SALT.

“We are competing with an established platform concept, and documentation is very important for the oil companies, which place high priority on safety.”

Eldøy is responsible for the design of the vessel, a very important element of which is the gangway, attached at the bow.

“The most important thing has been to improve the efficiency and safety of the connection between the offshore installation and the gangway, which is as much as 55 metres long. This is a critical point because the vessel must display the least possible vertical motion in response to the waves while at the same time maintaining its position,” he said.

The challenge has been met by incorporating features such as providing the vessel with a special hull shape, so that it looks almost the same at both bow and stern as whatever direction the waves come from, the motions of the vessel must be as small as possible.

The MARINTEK researchers also investigated the effectiveness of the azimuth thrusters in hotel mode. The direction and speed of each thruster is determined by a control system developed by Kongsberg Maritime receiving continuous input from GPS position measurements, among other things.

“We have also carried out a range of tests of the system by creating waves, wind and currents of different intensities in our Ocean Basin. During the model tests, Kongsberg’s DP system was linked to MARINTEK’s physical modelling and measurement systems. This enabled control of the model as if it were a full-scale ship,” said Alterskjær.

“Many specialised tests were also performed to investigate the interaction between the hull and the thrusters, and between the thrusters themselves. This gives Kongsberg more possibilities to tune and develop the control system.”

The hotel ship is being built by Hyundai Heavy Industries in South Korea under contract for Østensjø and there is an option for a second ship.